The Americans captured a 42-cm Big Bertha (together with a spare barrel) and carted her off to the proving grounds at Aberdeen. The gun was evaluated, then put on display outdoors for many years, along, I gather, with a Porsche tractor. I first became aware of this when I bought a copy of 'Museum Ordnance: The Magazine for the US Army Ordnance Museum' which carried an article on the subject, written by the late Konrad Schreier, in the November 1992 issue. Some of the photos I scanned, and can be seen in this online article:
The Museum Ordnance article was wrong in stating that the Aberdeen Bertha was scrapped during WW2 during a scrap drive - even more astonishingly, and indefensibly, it was sold to a scrap merchant in the early 1950s during a spring clean!!!
Those other infamous monsters, the Paris Guns, were, of course, spirited away back to Germany before the Armistice and scrapped so as to prevent anything falling into Allied hands. Probably the same happened to other big guns such as the various 28-cm field howitzers, and the one 30.5-cm field howitzer. At least one 38-cm railway gun survived and was in French hands, only to be recaptured by the Germans during the next world war.
The folks at Aberdeen kindly sent me a very hi-res scan of a hand-tinted photo in their possession of the Bertha in US hands - below is a small version: